Strange sketching behavior or just me being stupid?

I know STL Files are a pain to work with, but here comes my question: I imported a STL file, cut it in half and tried to draw a sketch on that plane (basically copying the cutline from the underlying Mesh Body). Problem was, there was no white point (sketching cursor) and therefore no sketching possible. Only when I move my cursor outside the mesh body section is sketching possible again…
So far so bad, after restarting my Mac (Mac Mini M2pro running Ventura) everything just worked again like a charm! I have experienced this behavior now over the last couple of days and I just can’t get my head around what the reason could be for this erratic behavior… Could you give me a hint?